Fluid driven handpieces have been used previously for many purposes and in reference to dental operations, they have been used for grinding, drilling, cutting, and polishing teeth and dental pieces. Among the controllers used by others for regulating the speed of such handpieces are those illustrated and described by James C. Martin in his U.S. Pat. No. 3,568,318 and by Ulf Martin Christian Apelskog and Lennart Erik Idoft Gidlund in their U.S. Pat. No. 3,567,330. In both of these previously designed controllers, the speed of the handpiece is controlled solely by regulating the release to the atmosphere of compressed air upstream of the air motor. As less of the compressed air is released, a greater air pressure is placed on the air motor and the greater is its speed. In Martin's patent the amount of air pressure on the air motor is controlled by the selectable movement of an operator's thumb over and against portions of an external bleed off orifice of an air line, otherwise delivering all the air to an air motor. When a load is applied on the handpiece during operation, a higher torque is produced by increasing the thumb coverage of the external bleed off orifice.
In Apelskog and Gidlund's patent, the air pressure applied to the air motor of the handpiece is preset, through the combined use of an air reduction valve and an electronically operated air bleed off control valve located upstream of the fluid motor. To prevent the speed of the handpiece from slowing down when a load is placed upon the forming tool being driven by the handpiece air motor, during its operation, magnets are attached to the rotating turbine of the air motor to initiate the creation of impulses monitored by the electronic control unit. It in turn operates to maintain the air turbine's rotational speed. In response to a reduced rotational speed, the electronic control unit will close the air bleed off control valve a proportionate amount, increasing the air flow and pressure directed to the air motor, thus producing a higher torque and returning the handpiece to its original higher speed.
Although these prior controllers result in the delivery of compressed air to provide a sufficiently high torque at high speed operations, at low speed operations there must be time allowed for the buildup of air pressure and flow to create the amount of torque and/or speed that is wanted. Therefore, there remained a need for a speed controller, which could provide compressed air to a handpiece to obtain greater available torque at low speed operation, and medium speed operation. This invention overcomes these disadvantages, by applying full air pressure to the handpiece at all start up times and thereafter during operations to obtain the higher torques needed in the low and median speed ranges, by using two valves, one valve controlling compressed air flow to the intake and the other valve controlling air flow from the exhaust of the air driven motor of the handpiece. Essentially at all times the method involves supplying full line pressure and the full volume air to the intake of the vane type air motor, and thereafter controlling its speed by regulating the flow of exhaust air leaving the air motor.